In recent years, in view of global environment problems due to exhaust gases, improvement in fuel consumption has been searched by lightening the body weights of transports such as automobiles. In particular, therefore, Al alloy forging materials of AA or JIS standard 6000 series (Al—Mg—Si) are used as structural materials or structural members of transports such as automobiles, particularly underbody parts such as upper arms and lower arms. Such 6000 series Al alloy forging materials have high strength, high toughness, and relatively excellent corrosion resistance. Also 6000 series Al alloy forging materials have excellent recycleability because of low alloy element contents and easy reusability of scraps as 6000 series Al alloy melting raw materials.
The 6000 series Al alloy forging materials are produced by homogenizing heat-treatment of an Al alloy cast material, hot-forging (die forging) such as mechanical forging or hydraulic forging, solution treatment, and then tempering including hardening and artificial aging. As forging materials, besides the cast material, extruded materials formed by extruding cast materials may be used.
Further, materials which realize high strength, high toughness, and high corrosion resistance are required for underbody parts such as suspensions. From this viewpoint, aluminum alloy forging materials have excellent strength and high reliability as compared with aluminum alloy cast materials.
In recent years, structural materials of such transports have been required to be further thinned and have higher strength and higher toughness in order to further lighten the weights of automobiles. Therefore, there are various attempts to improve the microstructures of Al alloy cast materials and Al alloy forging materials. For example, it has been proposed that the average grain size of crystals and precipitates of a 6000 series alloy cast material is decreased to 8 μm or less, and dendrite secondary arm spacing (DAS) is decreased to 40 μm or less in order to further improve the strength and toughness of an Al alloy forging material (refer to Patent Documents 1 and 2).
Also it has been proposed to further improve the strength and toughness of an Al alloy forging material by controlling the average grain size and average spacing of crystals and precipitates in crystal grains and on grain boundaries of a 6000 series Al alloy forging material. The control can improve corrosion resistance to intergranular corrosion and stress corrosion cracking. Further, it has been proposed to improve fracture toughness and fatigue properties by adding a transition element such as Mn, Zr, Cr, or the like, which has the effect of refining crystal grains, in order to refine crystal grains or make subcrystal grains in addition to the control of crystals and precipitates (refer to Patent Documents 3, 4, and 5).
However, such a 6000 series Al alloy forging material has the tendency to produce course crystal grains by recrystallization of a worked structure during the forging and solution treatment. When the coarse crystal grains are produced, high strength and high toughness cannot be achieved even by controlling the microstructure, and the corrosion resistance is decreased. In addition, in each of these patent documents, the forging work temperature is as relatively low as less than 450° C., and it is actually difficult to refine the intended crystal grains or form subcrystal grains by hot-forging at such a low temperature.
On the other hand, it has been known that in order to suppress the occurrence of coarse crystal grains due to recrystallization of the worked structure, a transition element having the effect of refining crystal grains, such as Mn, Zr, Cr, or the like is added, and hot-forging is started at a relatively high temperature of 450 to 570° C. (refer to Patent Documents 6, 7, and 8 to 10).
[Patent Document 1] Japanese Unexamined Patent Application Publication No. 07-145440
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 06-256880
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2000-144296 (Registration No. 3684313)
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 2001-107168
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2002-294382
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 5-247574
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 2002-348630
[Patent Document 8] Japanese Unexamined Patent Application Publication No. 2004-43907
[Patent Document 9] Japanese Unexamined Patent Application Publication No. 2004-292937
[Patent Document 10] Japanese Unexamined Patent Application Publication No. 2004-292892